A floating drilling unit or drilling rig on a fixed platform may be used to drill a well beneath water. A floating drilling unit typically is used in deep water areas while a fixed platform rig commonly is used in shallow water areas. In deep water drilling applications in which a floating vessel is employed, the vessel may be moored to the sea floor or may be kept in place with a dynamic positioning system. A marine riser consisting of multiple joints of large diameter steel pipe connected in series may be deployed from the vessel to a wellhead located at the sea floor.
In conventional drilling operations, the riser provides a conduit to return drilled earth cuttings from the well being drilled to the vessel, and to guide drilling tools from the floating vessel to the well. The primary drilling tools are a rotating drill bit that makes the hole and a drill string that conveys drill bit into the hole (wellbore) from the floating vessel. The drill string is essentially multiple joints of hollow steel pipe connected together that allows fluid flowing through it. It has a much smaller diameter than a marine riser, typically 3½ to 6⅝ inches in outer diameter. To cut the hole, the drill bit is either rotated by the drill string that is driven by a drilling machine at the vessel, or is rotated by a motor downhole in the drill string.
A fluid with predetermined density, called “drilling fluid” or “mud”, is injected down the drill string and out of the drill bit at the bottom of the hole. The mud carries earth cuttings from the bit upwards in the annular space between the drill string and the sides of the wellbore. Mud returns these cuttings to the vessel at the surface through the annulus between the drill string and the marine riser.
In conventional drilling, the wellbore is occupied by the drill string and a full column of mud from the surface all the way downwards to the bottom hole. The density of the mud must be designed in such a way that the hydrostatic pressure from the mud column is high enough to counter-balance the fluid pressure from the formation, referred to as “pore pressure”. Otherwise, formation fluids, typically hydrocarbons or water, will enter the well, and a discharge of formation fluids out of the well will occur. An uncontrolled discharge is known as a blowout that must be avoided. At the same time, the mud hydrostatic pressure in the wellbore must not exceed the rock strength, referred to as “fracture pressure”. Otherwise, if fracture pressure is exceeded, the formation surrounding the wellbore can be fractured. If fracturing occurs, the wellbore losses integrity and mud will flow out of the wellbore into the formation instead of returning back to surface, which is known as “lost returns” or “lost circulation”. In other words, wellbore pressure from the weight of mud column must stay within the operating window between formation pore pressure and fracture pressure at all times during drilling operations.
A principal challenge in deepwater drilling is to keep the wellbore pressure within the operating window long enough so that the potential hydrocarbon-filled pay zone may be reached. As the well deepens, the wellbore pressure will eventually be taken out of the required operating pressure window. A series of steel tubes called “casing” must be installed to reinforce the upper portions of the hole before drilling can continue with a smaller drill bit through the upper casing stings. As a result, the well diameter becomes progressively smaller as the well becomes deeper. The largest casing set immediately below sea floor is typically limited to 36 inches in diameter. The well must be able to reach the pay zone before it is required to run the smallest-size casing. Therefore, it is required to safely drill a hole section without casing (called “open hole”) for as long as possible.
As oil companies drill further offshore, large hydrocarbon deposits may be encountered. Pore pressure and fracture pressure are largely dependent upon the mass of the formations and seawater above that specific formation of interest. As water becomes deeper, however, the drilling operating pressure window between pore pressure and fracture pressure becomes narrower due to the higher percentage of seawater weight and lower percentage of rock weight above a zone. This narrow window can prevent drilling to the desired target depth. The challenge is even greater when a horizontal well is required. A horizontal well is known to increase oil and gas production by two to five times than a vertical straight well. Thus, it is highly desirable to drill horizontal wells.
The common practice in horizontal drilling is to line the wellbore walls of all the upper sections with casing strings before penetrating pay zones horizontally or near horizontally. Once the lateral hole is opened, no more casing can be installed before the lateral section is finished. The length of the lateral section through the pay zones must be long enough to achieve the desired production rate. The conventional drilling technology may be able to achieve the desired lateral length in a land well or a shallow water well where the operating window is wide enough. It typically does not work nearly as well in deep waters where the window is narrow.
Therefore, a need exists for a drilling system that can manage pressure so as to increase lateral, pay-zone wellbore length while staying below the fracture pressure.